1st if GPU is OC then revise to default value and run for few hours to check, If still the same problem then change check Sata Cable from PSU is okay not burned etc. then change riser sata cable then usb cable then riser.
Issue will be from above one of them. I believe
I had experienced this stuff when i do add up a card and troubleshooting up first on the PSU but i didnt resolve the issue. The problem is on sata cable is burned down.
OP, have you checked out those connections? Be sure not to miss a single one and be keen on burned parts.